1. Field of the Invention
The invention relates to a cylindrical alkaline storage battery, and more particularly, to a cylindrical alkaline storage battery adapted to prevent a short circuit between positive and negative electrodes.
2. Description of the Related Art
As alkaline storage batteries, there can be mentioned, for example, a nickel-cadmium rechargeable battery, a nickel-hydrogen rechargeable battery, etc., which contain different active materials. Among these alkaline storage batteries, there are cylindrical ones having a cylindrical outer can that accommodates therein an electrode group formed by strip-shaped negative and positive electrodes that are spirally wound with a separator interposed therebetween.
In this type of cylindrical alkaline storage battery, a short circuit can be caused between the positive and negative electrodes, and various proposals to prevent the short circuit have been made. For example, in case that a positive electrode obtained by cutting a sheet into a strip of a predetermined size with burrs on the cut ends thereof is rolled up together with a negative electrode and a separator to obtain an electrode group which is accommodated in an outer can, the burrs on the terminal end of the positive electrode can pierce through the separator disposed on the side close to the outer face of the positive electrode and come in contact with the negative electrode to produce a short circuit. In order to prevent the short circuit, an insulating protective piece is inserted between the terminal end of the positive electrode and the separator located on the outer face side of the positive electrode.
However, the terminal end of the positive electrode has not only burrs protruding outward in the diameter direction of the electrode group but also burrs protruding diametrically inwardly of the electrode group, resulting in a problem that a short circuit cannot sufficiently be prevented by the provision of the protective piece that is inserted between the terminal end of the positive electrode and the separator on the outer face side of the positive electrode. Specifically, the insertion of the protective piece causes the outer diameter of the electrode group to increase in the direction connecting the central axis of the electrode group and the terminal end of the positive electrode. Therefore, the electrode group is pressed by the rim and the circumferential wall of the outer can, particularly hard at the terminal end of the positive electrode when the electrode group is inserted into the outer can, and the burrs on the terminal end of the positive electrode protruding diametrically inwardly of the electrode group can pierce through the separator disposed on the side close to the inner face of the positive electrode and come in contact with the negative electrode, causing a short circuit.
The cylindrical alkaline storage battery is demanded to increase the battery capacity. By way of example in this regard, a cylindrical alkaline storage battery disclosed in Japanese Unexamined Utility Model Publication No. Sho 53-160720 has the positive electrode whose terminal end part is tapered toward the tip thereof to make the outer shape of the electrode group closer to a perfect circle, so that a clearance between the electrode group and the outer can is decreased, thereby increasing the volumetric efficiency to achieve high capacity.
However, the cylindrical alkaline storage battery disclosed in Japanese Unexamined Utility Model Publication No. Sho 53-160720 entails a problem that when the positive electrode is tapered by pressing or machining, a burr is produced on a ridge on the outer face of the terminal end part of the positive electrode, which pierces through the separator so disposed as to overlap the outer face of the terminal end part and causes a short circuit. Specifically, even in this cylindrical alkaline storage battery, the electrode group is not a perfect circle in cross section, and has its largest outer diameter in the direction connecting the central axis of the electrode group and the ridge of the terminal end part of the positive electrode. When the electrode group is inserted into the outer can, therefore, the terminal end part of the positive electrode is pressed hard at its ridge from both sides in the thickness direction by the rim and the circumferential wall of the outer can, to be compressed. As a result, the burr on the ridge pierces through the separator disposed on the outer face side of the positive electrode, to be in contact with the negative electrode, causing a short circuit.